Spectroscopic studies on the interaction of cationic surfactants with bovine serum albumin

The interaction of the cationic surfactant cetyltrimethylammonium bromide (CTAB) with bovine serum albumin (BSA), a globular protein, has been studied by small-angle neutron scattering (SANS), fluorescence and circular dichroism (CD). SANS measurements show that at low [CTAB] the protein shows a native-like behavior. On the other hand, at high [CTAB], surfactant molecules result in the formation of a fractal structure representing a ‘necklace model’ of micelle-like clusters randomly distributed along the polypeptide chain. The overall size of the complex increases and the fractal dimension decreases on increasing the surfactant concentration. The size of the micelle-like clusters does not show any considerable change while the number of such clusters and their aggregation number increases with increasing [CTAB]. Some extrapolatory experiments were performed with tetradecyltrimethylammonium bromide (TTAB) and the surfactant was found to behave similarly leading to an increase in the size of protein along the semi-major axis at low concentrations and formation of a fractal structure at high concentrations. The fluorescence studies undertaken were found to be consistent with the SANS measurements. Native-like behavior of the protein mixed with low concentration of the surfactant was also concluded from the circular dichroism (CD) spectra where the spectra in presence of high [CTAB] could not be monitored because of high dynode voltage.